1. Field of the Invention
This invention relates generally to an apparatus and method for use in orthopedic surgery and, more particularly, to an apparatus and method for positioning an orthopedic implant in a cavity in an intramedullary canal.
2. Discussion of the Related Art
A natural hip joint may undergo degenerative changes due to a variety of etiologies. When these degenerative changes become so far advanced and irreversible, it may ultimately become necessary to replace the natural hip joint with a prosthetic hip. When implantation of such a hip joint prosthesis becomes necessary, the head of the natural femur is first resected and a cavity is created within the intramedullary canal of the host femur for accepting the prosthetic hip. If the acetabulum also needs repair, all remnants of articular cartilage are removed from the acetabulum and a cup which will accommodate the head or ball of the prosthetic hip may be affixed to the acetabulum by means of cement, screws or other appropriate fixation means. The hip prosthesis may be inserted and supported within the host femur using generally one of two techniques.
These techniques either involve cementing the hip prosthesis within the host femur or using a non-cementing technique which generally uses a hip prosthesis which snugly fits within and is supported by the host femur.
When utilizing the cementing technique, it is generally desirable to position the hip prosthesis within the cavity made in the intramedullary canal so that a desired cement mantel is formed about the hip prosthesis. By providing a desirable cement mantel, this generally ensures that there are no regions within the cement mantel which may cause stress risers that may lead to subsequent fractures of the cement in these areas. If the hip prosthesis is not located in the desired position, the hip prosthesis may cause abnormal stress to the cement mantel and thereby lead to loosening of the hip prosthesis and the need for revision of the prosthetic hip. Various methods of positioning the prosthetic hip are known and used in the art. However, use of these various methods and techniques sometimes have several disadvantages.
For example, one method of positioning the hip prosthesis is to use a pair of sleeves made of high density polyethylene or polymethyl methacrylate for distal and proximal centering of the hip prosthesis within the cement mantel. One sleeve is positioned at the distal end of the hip prosthesis and the other sleeve is positioned at the proximal end of the hip prosthesis, thereby centering the hip prosthesis within the intramedullary canal. However, the disadvantage with this type of centering technique is that the distal and proximal sleeves are left in place as the cement hardens and may thus create stress risers in the cement mantel at these locations. The stress riser can be much more prominent in the proximal sleeve because this sleeve is generally larger than the distal sleeve.
Another centering method used incorporates a series of small bosses or pegs positioned about and integral with the periphery of the hip prosthesis at the distal and proximal portions. The bosses are used to center the hip prosthesis within the intramedullary canal similar to the sleeves. This technique, however, can also create stress risers in the cement mantel about the distal and proximal bosses.
An additional method utilizes a provisional prosthesis that carries a thumb screw. The thumb screw passes through the lateral portion of the provisional prosthesis and is used to engage the medial wall of the intramedullary canal. The provisional prosthesis is laterally centered within the intramedullary canal by appropriately adjusting the thumb screw. A transverse hole is then created in the host femur and a pin is inserted as close to the provisional prosthesis as possible. This pin is used to prevent the actual hip prosthesis from drifting medially as the implant is placed into the intramedullary canal. The disadvantages of this method are that it only provides control and centering in one direction or axis and may create a small stress riser in the cement mantel when the pin is subsequently removed after the bone cement is cured.
Still another method of positioning the hip prosthesis utilizes an integral collar positioned at the proximal end of the hip prosthesis. The collar nests with the calcar portion of the host femur that is exposed by resection of the head portion of the host femur. However, the integral collar hip prosthesis is generally more difficult to manufacture and therefore is more expensive to utilize than a collarless hip prosthesis.
What is needed then is an apparatus and method for positioning an orthopedic implant which does not suffer from the above mentioned disadvantages. This in turn, will help provide a desirable cement mantel about the orthopedic implant, reduce the number of possible stress risers within the cement mantel, provide implant positioning about multiple axes, reduce the amount of bone material removed from the host bone, and promote the use of lower cost implants, where appropriate. It is, therefore, an object of the present invention to provide such an apparatus and method for positioning an orthopedic implant.